📚 AQA A-Level Inorganic Chemistry: Periodicity, Group 2 and Group 7 | AQA A-Level 无机化学：周期性、第2族与第7族
In AQA A-level Chemistry, inorganic chemistry focuses on patterns across the Periodic Table, especially Period 3, Group 2 and Group 7. These topics link electronic structure to macroscopic properties such as melting point, ionisation energy, reactivity and acid-base behaviour.
在 AQA A-level 化学中，无机化学重点关注元素周期表中的规律，尤其是第三周期、第2族和第7族。这些主题将电子结构与熔点、电离能、反应活性和酸碱性等宏观性质联系起来。
1. Periodicity and the Periodic Table | 周期性与周期表
Periodicity is the repeating pattern of physical and chemical properties across a period. In Period 3, elements from Na to Ar show clear trends because the same 3s and 3p sub-shells are being filled while nuclear charge increases.
周期性是指元素物理和化学性质沿周期重复变化的规律。在第三周期中，从 Na 到 Ar 元素表现出明显的递变趋势，因为填充的是同一 3s 和 3p 亚层，而核电荷不断增加。
Across a period, the number of electron shells remains constant, so shielding is roughly unchanged. The increased attraction between the nucleus and outer-shell electrons explains many trends in atomic radius, ionisation energy and electronegativity.
在同一周期中，电子层数保持不变，因此屏蔽效应大致相同。原子核与最外层电子之间吸引力增强，这可以解释原子半径、电离能和电负性的变化规律。
The Periodic Table is divided into s-block, p-block and d-block elements. Groups 2 and 7, both in the s- and p-blocks respectively, are particularly important for AQA inorganic chemistry because their properties change in predictable ways.
周期表可分为 s 区、p 区和 d 区元素。第2族和第7族分别属于 s 区和 p 区，在 AQA 无机化学中尤为重要，因为它们的性质按可预测的方式发生变化。
2. Atomic Radius and First Ionisation Energy Across Period 3 | 第三周期原子半径与第一电离能
Atomic radius decreases from Na to Cl because the same-shell electrons experience a greater effective nuclear charge with little change in shielding. Ar is slightly different because it exists as a monatomic gas, but its atomic radius is still small.
从 Na 到 Cl，原子半径逐渐减小，因为同一电子层上的电子受到更大的有效核电荷吸引，而屏蔽效应变化很小。Ar 略有不同，因为它以单原子气体形式存在，但原子半径仍然较小。
First ionisation energy generally increases across Period 3. The general increase occurs because the outer electron is held more tightly by the increasing nuclear charge.
第一电离能沿第三周期总体增大。总体上升的原因是随着核电荷增加，最外层电子被吸引得更强。
However, there are two important dips. Aluminium has a lower first ionisation energy than magnesium because its outer electron enters a 3p orbital, which is slightly higher in energy than the 3s orbital. Sulfur has a lower first ionisation energy than phosphorus because its paired 3p electrons experience repulsion.
但有两个明显的下降点。铝的第一电离能低于镁，因为铝的最外层电子进入 3p 轨道，其能量略高于 3s 轨道。硫的第一电离能低于磷，因为 3p 轨道中的成对电子之间存在排斥。
The general order of first ionisation energies across Period 3 is: Na < Al < Mg < Si < S < P < Cl < Ar. This irregular order is a direct result of sub-shell energies and electron pairing.
第三周期第一电离能的一般顺序为：Na < Al < Mg < Si < S < P < Cl < Ar。这种不规则顺序直接由亚层能量和电子成对效应造成。
3. Melting Points Across Period 3 | 第三周期熔点的变化
Melting points across Period 3 show a sharp contrast between giant structures and simple molecular structures. Na, Mg and Al form giant metallic lattices; Si forms a giant covalent lattice; P₄, S₈ and Cl₂ form simple molecular structures; Ar is monatomic.
第三周期熔点表现出巨型结构与简单分子结构之间的鲜明对比。Na、Mg 和 Al 形成巨型金属晶格；Si 形成巨型共价晶格；P₄、S₈ 和 Cl₂ 形成简单分子结构；Ar 为单原子。
| Element / 元素 | Structure and bonding / 结构与键合 | Melting point trend / 熔点趋势 |
|---|---|---|
| Na, Mg, Al | Giant metallic lattice / 巨型金属晶格 | Increase from Na to Al because ionic charge increases and more delocalised electrons give stronger metallic bonding / 从 Na 到 Al 升高，因为离子电荷增加，更多离域电子使金属键更强 |
| Si | Giant covalent lattice / 巨型共价晶格 | Very high because many strong covalent bonds must be broken / 非常高，因为需要打断大量强共价键 |
| P₄, S₈, Cl₂ | Simple molecular / 简单分子 | Low, depending on van der Waals forces; S₈ is highest among these because it has the largest and most electron-rich molecule / 较低，取决于范德华力；其中 S₈ 最高，因为其分子最大且电子最多 |
| Ar | Monatomic / 单原子 | Very low because only very weak van der Waals forces exist between atoms / 非常低，因为原子间仅存在极弱的范德华力 |
These variations are a key piece of evidence for different structure types. You should be able to link melting point directly to the type of bonding and the strength of the forces that hold particles together.
这些差异是不同结构类型的重要证据。你应该能够将熔点直接与键合类型以及将粒子结合在一起的力的强度联系起来。
4. Group 2: Trends in Atomic Radius, Reactivity and Solubility | 第2族：原子半径、反应活性与溶解性趋势
Down Group 2, atomic radius increases and first ionisation energy decreases because the outer electrons are in higher energy levels with more shielding. This makes the elements easier to oxidise, so reactivity increases from Mg to Ba.
沿第2族向下，原子半径增大，第一电离能降低，因为最外层电子处于更高的能级，屏蔽效应更强。这使得元素更容易被氧化，因此从 Mg 到 Ba 反应活性逐渐增强。
Melting points generally decrease down Group 2 because the metal ions become larger and the delocalised electron density is spread over a greater volume, weakening the metallic bonding.
第2族熔点通常向下降低，因为金属离子变大，离域电子密度分布在更大的体积上，从而削弱了金属键。
Two solubility trends are particularly important. Hydroxide solubility increases down Group 2, so Mg(OH)₂ is sparingly soluble while Ba(OH)₂ is much more soluble. By contrast, sulfate solubility decreases down the group, making CaSO₄ only slightly soluble and BaSO₄ effectively insoluble.
有两个溶解性趋势特别重要。氢氧化物溶解度沿第2族向下增大，因此 Mg(OH)₂ 微溶，而 Ba(OH)₂ 溶解性大得多。相比之下，硫酸盐溶解度沿族向下降低，使得 CaSO₄ 仅微溶，而 BaSO₄ 几乎不溶。
BaSO₄ is used in medical barium meals because it is opaque to X-rays and very insoluble, so it does not release toxic barium ions into the body.
BaSO₄ 用于医学钡餐检查，因为它对 X 射线不透明且极难溶，因此不会在体内释放有毒的钡离子。
5. Group 2 Reactions with Water and Uses | 第2族与水的反应及用途
Magnesium reacts only very slowly with cold water, but it reacts readily with steam to form magnesium oxide and hydrogen:
镁与冷水反应非常缓慢，但它容易与蒸汽反应生成氧化镁和氢气：
Mg(s) + H₂O(g) → MgO(s) + H₂(g)
Calcium, strontium and barium react with cold water to form the corresponding hydroxide and hydrogen gas. For calcium, the equation is:
钙、锶和钡与冷水反应生成相应的氢氧化物和氢气。
Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图